Induction cooking unit having all pan safe operation, wide range power control and low start-up and shut-down transients

ABSTRACT

An induction heating unit including inverter circuit means comprised by a gate controlled thyristor and commutation circuit coupled together in circuit relationship and excited from a set of power supply terminals connected to a source of excitation potential. An induction heating coil is coupled to and excited by the inverter circuit means in a manner such that the loading on the induction heating coil determines at least in part the operating frequency at which the inverter circuit means operates. Gating circuit means comprised by a gating signal generator and enabling circuit means are coupled to and control turn-on of the gate control thyristor with the gating circuit means comprising a feedback sensing circuit coupled to the induction heating coil for deriving a feedback trigger signal synchronized with the frequency of operation of the commutation circuit.

United States Patent Peters, Jr.

PAN SAFE OPERATION, WIDE RANGE POWER CONTROL AND LOW STARTUP ANDSHUT-DOWN TRANSIENTS INDUCTION COOKING UNIT HAVING ALL a 1451 May27,1975 Primary Examiner-Bruce A. Reynolds Attorney, Agent, or FirmCharlesW. Helzer [75] Inventor: Philip H. Peters, Jr., Greenwich, [57] ABSTRACTNY. An induction heating unit including inverter circuit [73} Assign:Environment/One Corporation means comprised by a gate controlledthyristor and schfinecmdy N Y commutation circuit coupled together incircuit relationship and excited from a set of power supply termil lFiled? Apr- 26, 1973 nals connected to a source of excitation potential.An [2]] App], NO: 354 764 induction heating coil is coupled to andexcited by the inverter circuit means in a manner such that the loadingon the induction heating coil determines at least in CL 219/1049;219/1077; 321/l3 part the operating frequency at which the inverter cir-[Sl] Int. Cl. I'IGSb 5/04 uit means operates, Gating circuit meanscomprised [58] Field Of Search ..321/2, I8, 44, 14; by a gating ignalgenerator and enabling circuit 2 10.77 means are coupled to and controlturn-on of the gate control thyristor with the gating circuit means com-[56] References Cited prising a feedback sensing circuit coupled to thein- UNITED STATES PATENTS duction heating coil for deriving a feedbacktrigger 3 461 375 8/1969 Nestlen .1 H 321 43 Signal syr'chmnized withthe frequency of Operation of 3,553,567 1 1971 Pesce et al 321 14 thewmmulation drum- 3,566,243 2/1971 Landis 32l/l8 X 3,596,165 7/1971Andrews 321/18 x 29 24 Drawmg guns I6 R MCB r' J [14 l 123 [22 1 ,21 113 ZERO POINT ZERO POINT ll ALL r r 802-5 :1. DIODE PULSE comcmencr LEPOWER swncu C C 1411111 GENERATOR cmcun 1 A AND POWER comaunrmsRECTIFlER Low VOLTAGE or m gi's C gx 'EE NETWORK s con r am W CONTROL i1 SUPPLY 5mm 1 cmcu J 1 1 l 1 1 1 l l 1 29 1 1 z '1'" 1. L... .l 2 l 1 lI l l I 1 PAN L TEMP SENSOR AIR PATENTEMY 27 1915 SHEET A 21 m 5 e 5 m0.v o 8% I E, Q... E o r m Q Q 23 h F 3 h 1 a w v1 Swim v 0% 5. To to 1EM w: mm 7; EH 3 z 02 m v F, N a vT m J ma 5 M N? w NE 0 mm H 9; 3 T5 2m w L ay; 0 W J mm 5 lfi Ln f v H mm Q m 5 PATH-mum? mm 3,886; 342 SHEET3 ZERO POINT OPERATED TRIGGER MAIN CKT BKR ON, ARBITRARY TIME 2. 3 ifgamed o 0 0 m: 0 o o o 60 221| U U Ll U U 420 C3 (C)- cE 12 50ARBITRARYON W5 5 CLOSED ARBI RARY OFF TIME SZOPEN 2 CLOSED '1 d't f((1)9 05 11 V 5: z sg fifi dj 1L OEEN RUN PULSE OCCURS C5 BE 04 RF CYCLE1/1 PATENTED W27 I9'i5 SREEI 5 C /C POWER CONTROL WITH V 3 FEEDBACFSCR/DIODE CURRENT VS. TIME (NO-LOAD) TIME- lOps/DIV 2:; T 1 T 22:2 T a W1 u d,e 1s 34 52 (m' 2 0 e 17 2s 34 3 u,b 1e 22 38 (d) 4 u,b,c 20 2a 43N) i 6 .e- 23 23 4e NOT SHOWN FIG 5 SHEE? PATENTEUW 27 ms INDUCTIONCOOKING UNIT HAVING ALL PAN SAFE OPERATION, WIDE RANGE POWER CONTROL ANDLOW START-UP AND SHUT-DOWN TRANSIENTS BACKGROUND OF THE INVENTION Fieldof Invention This invention relates to a new and improved inductioncooking unit capable of safe operation with metal base pans of all typeshaving a wide range control of the power level at which the inductioncooking unit operates.

More particularly, the invention relates to an improved inductioncooking unit of the type intended for use with kitchen ranges having acool insulating top surface. At least one pancake-shaped planarinduction heating coil is disposed below the insulating surface andproduces magnetic lines of flux that are coupled to a pan or other metalbase cooking vessel placed on the insulating surface over the coil. Thecoil is excited by high frequency currents supplied to the coil by animproved chopper inverter supply constructed in accordance with thepresent invention, and which makes it possible to operate safelymetal-base cookware of all types including highly conductive aluminum,copper and other pans without endangering the induction cooking unit.Further the unit may be operated safely over a wide range of powerlevels including quite low power levels, of the order of onetwenty-fourth of the maximum level of power available from the unit, US.No. No.8c 3,710,062 issued Jan. 9, I973 for Metal Base CookwareInduction Heating Apparatus, etc., Philip H. Peters, Jr., Inventor,assigned to the Environment/One Corporation, of Schenectady, New York,describes an induction heating apparatus for metal base cookware. Theapparatus utilizes a solid state silicon control rectifier chopperinverter circuit to electrically excite a pancake shaped, planarinduction heating coil at comparatively high frequencies of the order ofkilohertz. The induction heating coil is disposed below a cooktop smoothinsulating surface on which metal base cookware to be heated is placed.Magnetic lines of flux produced by the induction heating coil aretightly coupled to and generate heat in the metal base cookware. Due tothe periodic build-up and collapse of the magnetic lines of flux at arelatively high rate of 20 Kilohertz, high frequency currents areinduced in the sur face of metal base cookware. Because of the highsurface resistivity of the metal base cookware at this high frequency,relatively efficient heating of the metal base cookware is achieved.

While the induction heating apparatus disclosed and claimed in US. Pat,No. 3,710,062 is satisfactory in many respects, it is desirable that theapparatus be capable of being safely operated with all types of metalbase cookware such as highily conductive pans of aluminum, copper, etc.,that the rf power produced by the apparatus be capable of beingcontrolled over a wide range of power including very low power levels;and that soft starting and shut-down of the induction heating unit tominimize high transient voltages and currents be incorporated in aninduction heating apparatus that is relatively simple and inexpensive tomanufacture and reliable in operation, and easy to maintain.

SUMMARY OF THE INVENTION A primary object of the present invention is toprovide a new and improved induction cooking unit which is capable ofall pan operation and which includes a pan safety control feature thatallows the unit to be operated safely with aluminum, copper or otherhighly conductive pans that inadvertently may be used by a housewife orother operator of the unit.

Another feature of the invention is the provision of a new and improvedinduction heating unit which is capable of operation over a very widerange of power levels including quite low power levels while stillproviding adequate commutating time intervals for the thyristor used inthe inverter circuit to excite the induction heating coil which arewithin the rate of recovery time of the thyristor.

A still further feature of the invention is the provision of the new andimproved induction heating unit having the above characteristics whichis capable of operation with a minimum stressing of circuit componentswith voltage and high transients.

In practicing the invention, a new and improved induction heating unitis provided which includes inverter circuit means comprised by a gatecontrol thyristor and a commutation circuit coupled together in circuitrelationship and excited from a set of power supply terminals that aredesigned for connection to a source of excitation potential that maycomprise a conventional residential, commercial or industrial source ofalternating current. An induction heating coil is coupled to and excitedby the inverter circuit in a manner such that the loading and unloadingof the induction heating coil determines at least in part the frequencyat which the inverter circuit means operates. Gating circuit means arecoupled to and control turn-on of the gate controlled thyristorcomprising a part of the inverter circuit means and comprises a feedbacksensing circuit means coupled to the induction heating coil for derivinga feedback trigger signal that is synchronized with changes in theresonant frequency of the commutation network. The gating circuit meansfurther includes a gating signal pulse generator for generating highfrequency signal pulses having a repetition rate determined by a desiredoperating frequency for the choppeninverter circuit and of sufficientenergy to insure turn-on of the gate control thyristor. Enabling meansare coupled to and enable initiation of operation of the gating signalpulse generator and is responsive to an alternating signal couplingcircuit means comprising a differentiating network which intercouplesthe last mentioned enabling means with the feedback sensing circuit forsynchronizing operation of the gating signal pulse generator withchanges in the resonant frequency of the commutating network of theinverter circuit due to loading and unloading of the induction heatingcoil.

Wide range control over the power output level of the inverter circuitmeans including operation at low power level, is achieved by switchingmeans for a plurality of commutating capacitor elements that comprisethe serially connected commutating capacitor reactive componentconnected in series circuit relationship with an inductor reactivecomponent to form the commutation circuit for the inverter. The gatecontrolled thyristor is connected in parallel circuit relationshipacross the serially connected capacitor and inductor commutatingreactive components. The capacitor commutating element switching meansis provided for connecting different values of commutating capacitancein operating circuit relationship in the commutation circuit means tothereby control the output power level generated by the circuit. Shuntcapacitor means are provided along with shunt capacitor switching meansfor switching certain ones of the commutating capacitor elements inshunt circuit relationship across at least a part of the inductorreactive component at low power levels whereby the shunt capacitorcomponents share current flowing through the inverter with the inductorcomponent coupled to the pan load at low power levels. This results inreducing the effective inductance of the inductive reactive componentand increasing the effective capacitance of the capacitor reactivecomponent so as not to reduce the circuit commutating turn-off timebelow a value which is less than the rated recovery time for the gatecontrol thyristor used in the circuit.

Startup and shut-down transients in the induction heating unit are keptto a minimum in a low cost construction by the provision of zero pointpulse generator means coupled to the power supply terminals forproducing an enabling output pulsed signal at the occurrence of eachzero point in the periodic undulating excitation potential supplied tothe inverter circuit means. Zero point coincidence circuit means arecoupled and control operation of the gating signal generator in responseto the output from the zero point pulse generator means so as tocondition the circuit for turn on as well as turn off of the gatingsignal generator means only at or near the periodic zero points in theperiodic undulating excitation potential.

These and other objects, features and advantages of the invention, willbe appreciated more readily as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings, wherein like V parts in eachof the several figures are identified with the same reference character;and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a functional block diagramof a new and improved induction heating unit constructed in accor dancewith the invention;

FIG. 2 is a detailed schematic circuit diagram of the new and improvedinduction heating unit shown schematically in FIG. 1;

FIG. 2A is a functional schematic sketch showing a preferredconstruction for the pancake-shaped heating coil disposed below a smoothand cool cooktop insulat ing surface and on which a pan or other metalbase cooking vessel is disposed during cooking operations;

FIG. 3(a) through FIG. 3(g) comprise a series of voltage wave fromversus time characteristic curves illustrating the manner of operationof the circuit shown in FIG. 2 at several points in the circuit;

FIGS. 4, 4A. 4B and 4C are functional schematic sketches showing theconstruction of the capacitor commutating reactive components and theswitching assembly employed to selectively connect such components inthe circuit shown in FIG. 2 to achieve wide range power control;

FIG. 5(a) through 5U) comprise characteristic wave shapes illustratingthe current through the thyristor power control device for varioussettings of the power control switching element shown in FIGS. 4Athrough 4C;

FIG. 6 is a detailed schematic circuit diagram of a low cost version ofthe improved induction heating unit circuit which does not employ zeropoint starting and turnoff; and

FIG. 6A is a schematic circuit diagram of a modification of the FIG. 6as well as FIG. 2 circuits and illustrates the manner of connection of apan temperature control to the circuits.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. I is a functional blockdiagram of a preferred form of a new and improved induction heating unitconstructed in accordance with the invention. In FIG. 1, an inductionheating coil L3 is shown at the right hand side of the drawing. and uponexcitation its near induced field can be magnetically coupled to a panll thereby causing the pan to be heated. A preferred phys icalconstruction for the induction heating coil L3 is shown in FIG. 2A ofthe drawings wherein it is seen that the coil L3 consists of onespirally wound coil or sev eral such coils stacked one above the otherto form a planar pancake. The coil L3 is disposed under an insulatedcool top surface 12 which physically supports the pan 11. Theundersurface of the insulating cool top surface (12) is provided with athin conductive coating or film (ESI) formed thereon by any suitabletechnique which is electrically grounded for high radio frequencies. Theconductive coating (ESI) thus forms an elec trostatic shield means forpreventing the build up of electrostatic charges on the cool topinsulating surface due to electrostatic coupling at the high radiofrequencies at which the power inverter operates. The desir ability andbenefits ofa cooking range utilizing such an induction heating coil andelectrostatic shield arrangement are described more fully in the abovereferenced U.S. Pat. No. 3,7l0,062, and in copending U.S. applicationSer. No. 263.639, and hence will not be repeated in detail.

The induction heating coil L3 is excited from an inverter circuit meansI3 that in turn is coupled to and excited from a pair of power supplyterminals 14 and 15 that in turn are adapted to be connected a source ofexcitation potential. The source of excitation potenial comprises a mainpower rectifier 16 that is designed to be connected to a conventionalsource of industrial. commercial or residential alternating currentthrough a fast acting magnetic circuit breaker MCB and operator controlswitch S3. The main power rectifier 16 preferably comprises a full waverectifier that supplies a full wave rectified potential across the powersupply terminals I4 and I5 to the chopper inverter circuit means 13whose construction will be described more fully hereinafter. Filteringof the high frequency component appearing between the power supplyterminals I4 and 15A is achieved with the filter and inverter charginginductor L2 in conjunction with filter-capacitor C2 connected acrossterminals [4 and 15. The full wave rectified I20 hertz potential of mainpower rectifier I6 appearing across terminals 14 and I5 also is filteredsomewhat (semifiltered) by the C2 filter capacitor.

The inter-relationship of the L2 inductor and the C2 filter capacitorand their role in the operation of the inverter circuit with asemi-filtered dc supply is described more fully in the above referencedU.S. Pat. No. 3.710.062.

Power control of the output power generated by the inverter circuitmeans I3 is achieved by CI/Co power control switch means l7 that isganged to operate in conjunction with an operator's on-off controlswitch 18, as indicated by the dotted line connection 19. Uponappropriate setting of the Cl/CO power control switch 17, and turn on ofthe inverter circuit means 13, the inverter will thereafter operate inthe normal manner of a series commutated chopper inverter to produceoutput power pulses of current having a frequency of the order of 20kilohertz which are supplied to the induction heating coil L3. For amore detailed description of the construction and operation of theinverter circuit means 13, reference is made to the above-noted US. Pat.No. 3,710,062, the disclosure of which hereby is expressly incorporatedby reference. Briefly, however, it can be stated that upon the siliconcontrol rectifier thyristor comprising a part of the inverter circuitmeans being gated on, the circuit automatically will discharge and thenrecharge a commutating capacitor with current flowing through theinduction heating coil L3, and thereafter will commutate (turn off) thethyristor. Repeated turn on and turn off of the thyristor in this manneroperates to produce the high frequency output pulses of current that areused to excite a current flow the induction heating coil L3 at the 20kilohertz rate.

Gating-on of the thyristor in the inverter circuit 13 is controlled byan all pan dv/dt t timing gating control circuit 21 which in turn iscontrolled at least in part by the operators on off control switch 18.The dv/dt t timer 21 is itself excited from a low voltage direct currentsupply shown at 20 which may derive its power from the main powerterminals 14 and 15 connected across the output from the main powerrectifier 16. Alternatively, the low voltage dc supply may be providedby the alternating current source from the line side of rectifier 16 viaa separate small transformer and rectifier arrangement in a conventionalwell known manner.

in operation the dv/dt r, timer produces gating on signal pulses havinga repetition rate related to the no load operating frequency of theinverter circuit 13. As will be explained more fully hereinafter, theloading and unloading of the L3 induction heating coil by the presenceor absence of pan 1] will affect the operating frequency of the invertercommutation circuit. In the case of highly conductive pans fabricatedfrom aluminum, copper, alloys of such materials and the like, the effectof the pan load is such that the commutating frequency of the invertercircuit is raised to a much higher value than it otherwise would be inthe absence of a load, or in the presence of a suitable, or proper loadsuch as a pan of iron, stainless steel or other lossy metal base cookingvessel. in order to follow changes in the commutating frequency of thecommutating circuit due to loading and unloading, feedback circuit meansin the form of a feedback resistor network including resistor R3, isconnected across the L3 induction heating coil, and supplies asynchronizing signal back to the a'v/dr t timer gating control circuit21 to lock its frequency of operation with the instantaneous operatingfrequency of the commutating circuit as is determined by its loadingcondition. As a result, changes in the commutating frequency of thecommutation circuit due to loading and unloading of the L3 inductionheating coil will not result in misfiring of the thyristor by the gatingcircuit with consequent improper operation of the inverter circuit aswill be appreciated more fully following a reading of the detaileddescription of FIG. 2 of the drawlngs.

In addition to the measure of control provided by the operators on-offcontrol switch 18, and the synchronizing signal from feedback resistorR3, the dv/dt t timer gating control circuit 21 also is controlled bythe output from a zero point coincidence circuit 22 that in turn iscontrolled by the output from a zero point sensing and pulse generatorcircuit 23. The zero point pulse generator circuit 23 is connected tothe main power rectifier 16, and produces a timing signal pulse upon theoccurrence of the each substantial zero point in the full wave rectifiedoutput potential appearing across the main power supply terminals 14 and15. The zero point timing signal pulse is supplied to a zero pointcoincidence circuit 22 which operates to condition the circuit forturn-on or turn-off of the dv/dt I, timer gating control circuit 21 onlyat or near substantial zero points of the undulating full wave rectifiedexcitation potential appearing across the power supply terminals 14 and15. By this means, the high frequency chopper inverter circuit means 13,is allowed to turn-on or turn-off only at or near the substantial zeropoints of the full wave rectified undulating excitation potentialappearing across main power supply terminals 14 and 15 thereby reducingto a minimum high voltage and a high current startup and turn-offtransients normally produced during start-up and turn-off of thecircuit.

In addition to the above features, a more sophisticated cooking rangemay include a pan temperature sensor and amplifier control circuit 24.The pan temperature sensor employs an infra red temperature sensor whichviews the bottom of pan 11 and thereafter operates to control turn-onand turn-off of the dv/dt t timer gating control circuit 21 whereby pan11 may be maintained at some desired preset temperature level. For amore detailed description of the sensor amplifier 24, reference is madeto [1.5. Pat. No. 3,7l0,062 and to FIG. 6A described hereinafter.

In operation, after the circuit of FIG. 1 is turned-on by the housewifeor other operator of the induction heating unit and set for achieving adesired amount of heating of the pan 11 as determined by the setting ofthe C1/C0 power control switch 17, at which point the line switch S3 andoperators on-off control switch 18 have been closed in the sequencenamed to commence operation of the circuit. It is assumed that thecircuit breaker MCB stays closed unless tripped by a fault. The fact ofclosure of the operators on-off control switch 18 is not of itselfsufficient to initiate operation of the inverter circuit 13 for that cannot occur until such time that a zero point is detected by the zeropoint pulse generator 23. Upon this occurrence the zero pointcoincidence circuit 22 conditions dv/d! t timer gating control circuit21 to initiate operation. Thereafter the gating control circuit 21 willgenerate output gating pulses at a frequency synchronized with thefrequency of operation of the inverter circuit commutation network asdetermined by the loading and unloading on the L3 induction heating coilas described above.

While the dv/dt r timer gating control circuit 21 is precondition foroperation by closure of the operators on-off control switch 18 with anappropriate presetting of the C1/C0 power control switch, actualoperation does not take place until the occurrence of a zero point ofthe undulating full wave rectified excitation potential appearing acrosspower supply terminals 14 and 15. By reason of this feature, start-upvoltage and current transients are maintained at a minimum. By reason ofthe synchronized feedback through the feedback resistor R3 to controlturn-on of the dv/dt t timer gating control circuit 2]. proper and safeoperation of the inverter circuit 13 under all conditions ofloading andfor all types of pans, is assured. Finally. the design of the Cl/Ct)power control switch 17 as described hereinafter provides a large rangeof power control of the output control produced by the inverter circuit[3 including very low levels of output power such as is required incooking applications and yet allows safe operation of the invertercircuit over the full range of output power levels. improved FIG. 2 is adetailed schematic circuit diagram of the new and iproved inductionheating unit constructed in accordance with the invention. In FIG. 2, itwill be seen that the induction heating coil L3 is connected in seriescircuit relationship with a capacitor C3 with the series circuit thuscomprised connected in parallel circuit relationship across acommutating ca pacitor C1. The commutating capacitor C1 in turn isconnected in series circuit relationship with a commutating inductor Lland the series circuit thus comprised is connected across the powersupply terminals 14 and 15A. Power supply terminals 14 and ]5A aresupplied with the undulating, full wave rectified excitation potentialsupplied from the output of the main power rectifier l6 comprised by adiode bridge D4, D5, D6 and D7 connected in a conventional full waverectifier bridge network. A power rated thyristor comprised by a siliconcontrol rectifier (SCR) Q1 has its anode connected to the positivepolarity power supply terminal 14 and its cathode connected to thenegative polarity power supply terminal 15A. A feedback diode D1 isconnected in reverse polarity, parallel circuit relationship with theSCR Q1. A suitable current sensing resistor R27 is connected in serieswith Coil L3. A conventional snubbing network comprised by a capacitorC7 and series connected resistor R17 is connected in parallel across theSCR Q] and feedback diode D1. A pilot SCR O2 is connected across themain power SCR Q1 through a current limiting resistor R15 and a commoncoupling resistor R16 that serves to transfer a gating-on pulse ofsufficient magnitude to the control gate ofQl to assure safe turn-on ofQ1 even at very low levels of semi-filtered rectified supply potentialacross terminal 14 and 15A of the order of 10 volts.

ln operation, upon the Q2 pilot SCR receiving a gating on pulse from thedv/dr t timer gating pulse generator 21 to be described hereinafter 02turns on and couples a gating on pulse through the common load resistorR16 to the control gate of the main power thyristor 01. It is assumed atthis point in the operation, that the circuit breaker MCB has beenclosed as well as the 83 line switch and that excitation power from thefull wave rectifier 16 is being supplied to power supply terminals 14and X5. Under these conditions the commutating capacitor C1 (as well asthe filter capacitor C3) will be charged substantially to the averagerectified line voltage Vs of the power supply terminals 14 and 15.Ignore for the time being the shunt capacitor C0.

Upon gating on of the main power SCR O] the charge on the commutatingcapacitor C] will be oscillated through the conducting SCR Q1 producinga current flow through the commutating inductor which at the end of onehalf 1, cycle will cause the polarity of the charge on the commutatingcapacitor C] to reverse. Thereafter. the reverse polarity charge on thecommutating capacitor C1 will cause current to flew back through theinductor L1 and reverse connected feedback diode D1 to return the chargeon capacitor C1 to substantially its initial polarity and chargedcondition minus any circuit losses due to loading, etc. During thisreverse conduction interval through feedback diode D1, the reversepolarity across SCR ()1 will cause it to commutate off (turn off) sothat upon completion of the oscillation and return of the Cl commutatingcapacitor to substantially its original charged condition. current flowthrough the DI feedback diode will be blocked and the circuit willassume its original charged condition ready for another pulsedoscillation. It will be appreciated that the interval of time in whichthe feedback current flows back through the feedback diode D1 determinesthe amount of commutating turn off time available in which to turn offSCR ()1.

Because of its nature and design the SCR 01 requires some predeterminedminimum turn off time in order to assure that it will regain its currentblocking capabilities well in advance of the forward potential appearingacross the power supply terminals 14 and 15A being reapplied across itupon the feedback diode D1 ceasing conduction. Because of thisrequirement, the C0 shunt capacitor, and its appropriate switchingcircuitry S0 (described hereafter with relation to FIG. 4), has beenincluded to assure adequate turn off time for the SCR 0] while operatingunder low power output conditions as will be explained more fullyhereinafter in connection with FIG. 4 of the drawings.

The above brief description of the chopper inverter circuit operationignores the effect of the induction heating coil L3, which inconjunction with its series connected smoothing capacitor C3 functionsalso as a smoothing filter to produce an essentially sinusoidal waveform current flowing through the L3 induction heating coil as explainedmore fully in the above referenced US. Pat. No. 3,710.062. The value ofinductance of the L3 induction heating coil and the value of capacitanceof capacitor C3 are adjusted so that the reactance of the parallelnetwork comprised by L3, C3 and Cl always remains capacitive in natureat any frequency at which the inverter oscillates. The frequency ofoscillation of the charge on the Cl commutating capacitor is determinedprimarily by the net capacitance of the parallel combination of thecapacitor Cl and the reflected capacitance of the L3 C3 C0 network in series with the inductance of L1. This frequency, known as the commutatingfrequency, is changed by loading and unloading of the L3 inductionheating coil upon a pan ll being brought into inductive relationshipwith L3 or removed from it. Hence. loading and unloading of the L3induction heating coil results in changing the natural resonantfrequency of the entire commutating network, To accommodate this changein resonant frequency of the commutating network due to loading andunloading in a relatively simple and inexpensive manner, the presentinvention was devised.

Briefly stated. dynamic changes in the resonant operating frequencyofthe commutating network caused by tuning and detuning of the L3induction heating coil due to loading and unloading with a pan, areaccommodated by appropriate synchronization of the operation of thegating pulse signal generator 2] with a synchronizing feedback signal.

Synchronization of the operation of the gating pulse signal generator 21is achieved by means of a synchronizing feedback signal derived from afeedback circuit means comprised by a pair of sensing resistors R2 andR3 connected in series circuit relationship across the L3 inductionheating coil. The junction of the sensing resistors R2 and R3 is coupledthrough alternating current signal coupling circuit means comprised by acapacitor C4 coupled to the base electrode of an enabling switchingtransistor Q4. Capacitor C4 and mainly resistor R2 comprise adifferentiating network for differentiating the synchronizing feedbacksignal VL3 derived from across the induction heating coil L3, and applythe differentiated signal to the base electrode of the switchingtransistor Q4. A biasing resistor R4 is connected between the base of Q4and the power supply terminal 14 and a capacitor C4 is connected inparallel across the R4 biasing resistor and integrates so as to delay orprolong the turn-on period of conduction for the transistor Q4.

The switching transistor Q4 comprises an enabling means for enablingoperation of the gating pulse gener ator signal 21. Gating pulse signalgenerator 21 is comprised by a programable unijunction transistor 03(hereinafter referred to as a PUT). PUT Q3 has its anode connectedthrough a variable resistor R7 and a fixed resistor R36 to the collectorof PNP enabling switching transistor Q4 whose emitter is connected topositive power supply terminal 14. The cathode of PUT O3 is connectedthrough the primary winding of a pulse transformer T1 to the negativeterminal 25 of a 20 volt low voltage direct current power supply 20.

The low voltage direct current power supply 20 is comprised by a zenerdiode Z1 having a filter capacitor C8 and filter resistor R8 connectedin parallel across it with the circuit thus comprised being connected inseries circuit relationship with a voltage dividing resistor R1 acrossthe main power supply terminals 14 and 15. As a consequence of thisarrangement, as voltage builds up on the main power supply terminals 14and 15, a corresponding self derived direct current low voltage of 20volts will be produced across the low voltage power supply terminal 25relative to the main power supply terminal 14. This low voltage 20 voltdirect current supply provides excitation potential for all of thegating subcircuits comprising a part of the induction heating unit. Itis believed obvious to one skilled in the art that a separate lowvoltage direct current supply could be provided by means of a separateac transformer and rectifier arrangement connected to the input acsupply.

The gating pulse generator 21 is further comprised by a timing capacitorC which is connected in parallel circuit relationship with PUT Q3 andprimary winding of transformer T1 and in series with the resistor R7 andfixed resistor R36. A pair of biasing resistors R9 and R are connectedin series circuit relationship across the serially connected resistorsR7 and R36 and PUT ()3 with the juncture of the biasing resistors R9 andR10 being connected to the anode gate of PUT 03. Upon switchingtransistor Q4 being turned on, timing capacitor C5 will be chargedpositively from the volt supply through resistors R36 and R7 at a ratedetermined by the overall time constant of C5 and resistors R36 and R7.This rate is adjustable through the medium of variable resistor R7.Assuming no other interaction or interruption of the charging of timingcapacitor C5, charging of C5 will continue until it will reach a valuewhere it will cause PUT O3 to breakdown and conduct at a charging leveldetermined by the value of the biasing potential supplied to the anodegate of PUT Q3 from the biasing resistors R9 and R10.

FIG. 3(3) of the drawings illustrates the buildup of charge on timingcapacitor C5 under conditions that provide an overall 1 time at the endof which a gating pulse will be produced across the primary winding ofpulse transformer T1. The gating pulse is supplied by the secondarywinding of Tl across load resistor R38 and applied to the gatingelectrode of the Q2 pilot SCR thereby to initiate one oscillation of thechopper inverter circuit as described previously. At this point in thedescription it would be good to note that as the charge on the timingcapacitor C5 builds up towards its ultimate firing value determined byR9 and R10, should conduction of the enabling transistor 04 beterminated, the biasing potential to the anode gate of PUT Q3 frombiasing resistors R9 and R10 will disappear and PUT Q3 automaticallywill conduct to produce a gating on pulse at some intermediate pointindicated at in FIG. 3(g) of the drawings. Thus. it will be appreci atedthat control of the buildup of charge on the timing capacitor C5 by turnon and turn off of the enabling transistor Q4, provides control over thetiming of production of the t timing pulse and is exercised by turningoff the enabling transistor Q4 at some intermediate point short of thefull 1 time determined by the RC time constant of C5 and R36, R7.

In order to control turn on and turn off of the induction heating unitan operators on off control switch S2 is provided. Switch S2 is operatedonly after a line switch S3 connected in the AC line side of the fullwave rectifier power supply 16 has been closed through me chanicalinterconnection of the switches to provide power to the low voltage DCpower supply 25. Switch S2 is ganged to and operates in conjunction withthe Cl/CO capacitor switching arrangement S1 and S0 which will bedescribed hereinafter. Switch S2 is connected in series circuitrelationship with a normally closed contact of an over temperature limitswitch 26 and with a resistor R5 across the low voltage DC power supplyterminals 14, 25. The normally closed over temperature limit switch 26may comprise a bi-rnetal tem perature sensitive switch, PTC thermistoror other thermally sensitive element which is physically disposedadjacent to a current sensing resistor 27 connected in series circuitrelationship with the induction heating coil L3. The current sensingresistor 27 will heat up in the presence of excessive current flowthrough the induction heating coil L3 and will cause the bimetal orother normally closed contacts to over temperature switch 26 to open tothereby interrupt operation of the induction heating unit in the samemanner as will be described hereinafter when the S2 operators controlswitch is opened. Alternatively. the thermally sensitive element couldbe arranged to sense the temperature of the L3 induction heating coil.the temperature of the insulating counter top, or other part whereminimum temperature levels during of operation are to be maintained.

Series resistor R5 is in effect part of the voltage dividing resistornetwork comprised by resistor R5, resistor R13 and resistor R14 thejuncture of R5 and R13 being connected through normally closed switchcontact 26 and the operators control switch S2 down to the negativeterminal 25 of the low voltage direct current power supply. The junctureof resistors R13 and R14 is connected to the base electrode of an NPNswitching transistor Q having its emitter connected to the power supplyterminal 25 and having its collector connected in series circuitrelationship with a pair oflimiting resis tors R11 and R12 to the powersupply terminal 14. The juncture of resistors R11 and R12 is connectedto the base electrode of a PNP switching transistor Q6 whose emitter isconnected to the power supply terminal 14 and whose collector isconnected through a limiting resistor R13A back to the base of the NPNswitching transistor Q5. Thus it will be appreciated that the resistorsQ5 and Q6 are connected in a hooked manner so that turn on of thetransistor Q5 results in turn on of transistor Q6 which further feedsback to the base of Q5 and latches it on in a conducting condition. Thispositive latching on and off of Q5 and Q6 switching transistors locksout any transient voltage spikes that might be caused by erraticoperation of the operator switch S2 and/or normally closed overtemperature switch 26.

In the circuit of FIG. 2, closure of the on off opera tors switch S2 isnot of itself sufficient to initiate operation of the induction heatingunit chopper inverter 13 because of the inclusion of a zero pointstarting and stopping control in the arrangement. Zero point controlover the operation of the chopper inverter is achieved by means of azero point pulse generator shown generally at 23 and comprised by aZener diode 22 connected between the power supply terminal 14 andthrough a limiting resistor R19 to the common connected anodes of a pairof diodes D9 and D10. Diodes D9 and D are connected across the dioderectifier bridge D4 through D7 in a manner so as to assure the absenceof a potential across Zener diode Z2 and its corresponding load resistorR20 only upon the occurrence of the recurring zero point in theundulating, unidirectional, semifiltered full wave rectified excitationpotential appearing across the output terminals of the full waverectifier bridge D4 through D7. On either side of the recurring zeropoints, either D9 or D10 will be conductive and will result in theproduction of a negative polarity bias potential across Z2 and loadresistor R20. This negative bias potential is supplied through limitingresistor R21 to the base electrode of a PNP switching transistor 07.Transistor Q7 has its emitter connected directly to power supplyterminal 14 and its collector connected through a limiting resistor R22to the negative terminal 25 of the low voltage direct current powersupply. The collector of Q7 also is connected to the base of a secondPNP switching tansistor 08 having its emitter connected to the powersupply terminal 14 and having its collector connected to a commonswitching terminal 28 of the zero point pulse coincidence circuit 22.

in operation, on either side of the zero point of the full waverectified excitation potential supplied across the main power supplyterminals 14 and 15 as shown in FIG. 3(a). a bias potential will bedeveloped across Zener diode Z2 and load resistor R20 which keepsswitching transistor 07 turned on and conducting. With Q7 turned on apositive polarity bias potential will be applied to the base of thesecond switching transistor Q8 which maintains this transistor turnedoff. How ever. upon the occurrence of a zero point in the full waverectified excitation potential, no bias potential appears across the Z2Zener diode and its load resistor R20 and Q7 turns off. Hence, O8 isallowed to turn on due to a negative bias potential supplied to its basethrough a resistor R22 from the 20 volt negative terminal 25 of the lowvoltage direct current power supply 20 as shown in FIG. 3(h), (b).

The zero point pulse coincidence circuit 22 is comprised by a pair ofPNP switching transistors 09 and Q10 having their emitter electrodesconnected in common to a switching terminal bus bar 28 that is excitedonly in response to the switching transistor 08 being turned on. Theemitters of switching transistors Q9 and Q10 are connected through thelimiting resistors R23, R24 and R30 and R31 respectively, to the 20 voltlow voltage direct current power supply terminal 25. The juncture of theemitter resistors R23 and R24 is connected to the base electrode of anNPN switching tran' sistor Q11 having its emitter connected through acommon cathode load resistor R29 to the low voltage power supplyterminal 25. Common cathode load resistor R29 also is connected to theemitter of a second NPN switching transistor Q12 in a manner such thatthe transistors Q11 and Q12 comprise a bistable multivibrator havingcommon cathode coupling and two stable states of operation. Thecollector Q11 is connected through a limiting resistor R25 to the mainpower supply terminal 14 and through a coupling resistor R27 back to thebase of the Q12 transistor, and the collector of Q12 is connectedthrough a limiting resistor R26 to the main power supply 14 and througha resistor R28 back to the base of the transistor Q11. The collector Q11also is connected through a limiting resistor R33 to the junction offeedback sensing resistors R2 and R3 for a purpose to be describedhereinafter.

From the above description it will be appreciated that when the latchingtransistor Q5 is turned on, the base of Q10 will be clamped throughresistor 32 to the potential of the low voltage negative power supplyter minal 25 thereby rendering transistor Q10 conductive. Simultaneouslybecause of the latching turn on action through feedback to the base ofQ6 by resistors R11 and R12, Q6 will be rendered conductive and willclamp the base of NPN transistor Q9 through resistor R35 to the positivepolarity potential power supply ter minal 14. Thus, it will beappreciated that positive switching action of the operation oftransistors 09 and Q10 is obtained in response to the conduction ornonconduction of the latching transistors Q5 and Q6. With transistor Q12switched on, a lock out condition is imposed on the charging of thetiming capacitor C5 due to capacitor C5 being clamped very near thepotential of terminal 25 through a diode D3 and conductor 29 and theemitter collector of transistor Q12.

As described earlier, prior to operator control switch S2 being closedand with line switch S3 closed, transistors Q5 and Q6 are latched on inthe conducting condition due to a positive polarity turn on potentialapplied to the base of Q5 by resistors R5, R13 and R14. Con ductionthrough Q5 connects the base of switching transistor Q10 to the negativeterminal 25 of the low voltage direct current power supply therebycondition Q10 for turn-on, and upon the occurrence of a zero point inthe supply potential across power supply terminals 14 and 15, Q10 isallowed to conduct. Conduction through Q10 applies a positive polarityturn on potential to the base of Q12 rendering Q12 conductive andlocking out or blocking conduction of Q9 and Q11 by reason of the commoncathode coupling through resistor R29 and the feedback to the base of 29from the collector of Q12. With Q12 conducting, timing capacitor C isclamped to the potential of terminal 25, and gating signal pulses cannotbe generated for application to the control gate of pilot SCR Q2. Thusoperation of the induction heating unit is inhibited.

Thereafter, should the operators on off switch S2 be closed by ahousewife or other operator of the circuit, the base of Q5 will bedriven negative toward the potential of terminal 25 causing O5 to turnoff and this in turn causes O6 to turn off in the previously describedmanner. This results in applying a negative turn on signal to the baseof transistor 09 causing it to turn on at the next succeeding zeropoint. Turn on of ()9 under these conditions will apply a positive turnon potential to the base of OH to cause it to turn on and to turn-off012 due to the common cathode coupling through R29 as shown in FIGS.3(0) and 3(d). Turn off of Q12 is assured due to 010 being maintainedoff at the current zeros by reason of the positive polarity inhibitpotential at its base with 05 off. Turn-off of Q12 removes the clampacross the timing capacitor C5 through coupling diode D3 and conductor29 thereby allowing C5 to charge and conditions the gating pulsegenerator to commence operation.

For some arbitrary period after the on off switch S2 has been closed thecollector voltage of 012 remains at the volt level of low voltage DCpower supply terminal as shown in FIG. 3(a). However, upon theoccurrence of the next successive zero point of the supply potential thecollector voltage Vce of Q12 rises to ward the potential of terminal 14and conduction through Q12 ceases. At the same instant Q11 turns-on andthe collector of Q11 drops to near the 20 volt level of DC power supplyterminal 25 as shown in FIG. 3(d). This results in the production of aninitial gating on signal pulse that is supplied through the resistorR33, conductor 31 and differentiating capacitor C4 to the base of theenabling transistor Q4. Turn on of Q4 results in the production of agating on signal pulse from the dv/dl t timer gating circuit 21 and issupplied through pulse transformer T1 to gate on pilot SCR Q2 and themain power SCR Q1 as described previously to initiate sustained chopperinverter oscillations.

FIG. of the drawings illustrates schematically the nature and generalwave shape of the high frequency oscillatory output of the chopperinverter appearing across the main SCR between terminals 14 and ISA andmodulated with the envelope of the full wave rectifier excitationpotential appearing across the main power supply terminals 14 and 15.The high frequency oscillatory output is produced as a result of thehigh frequency t gating pulses shown in FIG. 3(e) and supplied to pilotSCR 02 by the dv/d! t2 timer 2] through pulse transformer T1. FIG. 3(g)of the drawings illustrates the wave shape of the feedback synchronizingpotential VR appearing across the sensing resistor R2 at a point in timedifferent from the wave shapes shown in FIGS. 3A through 3F and at apoint in one oscillatory cycle where this voltage swings from negativeto positive polarity and then back through the zero point to assume anegative polarity. This VR2 potential is substantially in phase with thepotential VL3 of heating inductor L3, and is used to initiate the startof the 1., charging time of capacitor C5. The positive going half cycleof the VR potential shown in FIG. 3(3) corresponds to the conductioninterval 1 of the main SCR Q1 and power feedback diode D1. As the VR2potential passes through zero and starts to go negative and a current isinjected into the base of Q4 by the differentiating action of capacitorC4 causing O4 to turn on and initiate the t charging time as describedpreviously. Thereafter, due to the smoothing action of the capacitor C4and resistor R4, Q4 will be maintained on and conducting over the tcharging interval and timing capacitor C5 will be charged towards thepositive polarity of the DC supply potential 25 in the manner indicatedby the increasing ramp voltage marked VCS. If charging of C5 weredetermined only by the RC time constant of the circuit, C5 would becharged for the full pe' riod t2 time indicated in dotted outline, formin FIG. 3(g). However, the VR2 voltage which tracks the voltage acrossthe L3 induction heating coil will reach some maximum negative value andthen starts toward zero in the positive going direction. Hence, thedifferentiated value of VR2 likewise will go positive so that the baseof O4 is driven positive relative to its emitter and Q4 will turn offsharply. Turn off of Q4 removes the inhibiting potential supplied to theanode gate of 03 by voltage dividing resistors R9 and RIO so that atthis point PUT Q3 fires and produces a gating on signal pulse which issynchronized exactly with the natural resonant frequency of thecommutating network. It is this operating characteristic that gives thecircuit its frequency pushing capability so that as tuning and detuningof the commutating network occurs clue to placement and removal, of panloads, synchronization of the gating pulses shown in FIG. 3(e) withchanges in the natural resonant frequency of the commutating network isassured.

In the presence of an aluminum, copper or other highly conductive pan,the natural commutating frequency of the chopper inverter increases dueto the effect the highly conductive pan has in lowering the inductancevalue of the induction heating coil L3. Consequently, the frequency ofthe synchronizing pulses appearing across the feedback sensing resistorsR3 and R2 and coupled to the base of Q4 through differentiatingcapacitor C4, likewise will increase corresondingly to assuresynchronization of the generation of the gating signal pulses with thechanges in frequency due to such loading.

If as a consequence of this condition, current flow through the L3induction heating coil becomes exces sive, the excessive current will besensed by the current sensing resistor 27 and thermally sensitivecontacts 26 shown in FIG. 2A. Opening of thermally sensitive contacts 26will turn off the induction heating unit for a short interval to allowthe unit to cool in a'manner to be described hereinafter and then turnback on auto matically in a cyclic fashion.

In addition to assuring soft turn on of the induction cooking unit atsubstantially only the zero points of the undulating supply excitationpotential appearing across power supply terminals 14 and 15 the zeropoint coincidence circuit 22 also functions to assure soft turn off ofthe unit upon an operator causing the operators on off switch S2 to bemoved to the off or open position, or upon opening of the thermallysensitive over temperature contact 26. Additionally, should a pantemperature and sensor amplifier unit 24 shown in block diagram form inFIG. 1 call for turn off of the induction heating unit for pantemperature controlling purposes, such turn off can be made to occur atonly the substantial zero points of the undulating supply excitationpotential appearing across terminals 14 and 15. This zero point turn offis achieved by means of the clamp placed across the timing capacitor Cof the dv/dt t timer gat ing control circuit 21 through diode D3.

Assume that the induction cooking unit has been operating for some timeand that the housewife operator causes the on off switch S2 to be movedto its off position. Upon this occurrence, a positive polarity turn onpotential is applied to the base of transistor Q5 causing Q5 and itslatching transistor O6 to turn on and latch on in the conductingcondition. It will be noted in FIG. 2 that the collector of O5 isconnected through resistor R32 to the base of transistor Q and thecollector of O6 is connected through R35 to the base of transistor 09 ofthe zero point coincidence circuit 22. Thus, it will be seen that uponQ5 and Q6 being rendered con ductive upon opening of switch S2, O9 isconditioned to conduct. However, assuming that opening of the S2 switchoccurs at some intermediate supply voltage level between the zero pointsof the undulating supply excitation potential appearing across powersupply terminals l4 and 15, then Q9 and Q10 cannot conduct due to thelack of an enabling potential on their emitter electrodes. However, uponthe occurrence of the next successive zero point, the zero point pulsegenerator 23 will turn on transistor Q8 thereby allowing transistor Q10to turn on while transistor O9 is maintained off due to the positivepolarity bias potential supplied to its base by 06. This results incausing the bistable multivibrator comprised by 011 and Q12 to beswitched whereby 012 is conducting and Q11 is maintained off by reasonof the common cathode coupling through the resistor R29. With transistorQ12 conducting, a clamp will be maintained across the timing capacitorC5 in the gate pulse generator 21 through diode D3 whereby preventingthe gating pulse generator from producing gating on pulses. Since thisoccurs at only the substantial zero points in the undulating excitationsupply potential appearing across power supply terminals 14 and 15, softturn off of the chopper inverter circuit is assured thereby reducing toa minimum high transient voltages and currents through the circuitcomponents during the shutdown process.

FIGS. 4, 4A, 4B and 4C illustrate in greater detail a preferredconstruction for the power controlling capacitor switching arrangementused with the chopper inverter circuit. It should be noted that whilethe capacitor switching arrangement herein described is preferred forpower controlling purposes, other arrangements such as using a separatevariable inductor L3 in series with the induction cooling coil L3, couldbe used satisfactorily for power control purposes. FIG. 4A of thedrawings illustrates a plurality of capacitor ele ments 0, b, c, d, e,f,which are connected to a common B+ power supply terminal that in factwith the circuit of FIG. 2 could constitute the power supply terminal14. The upper end of the capacitor elements are shown connected to a rowof contact points numbered 1 through 10 which constitute one set offixed contacts points of a two deck rotary switch of conventional,commercial construction. FIG. 4 of the drawings is a schematicillustration of the manner in which the wiper contacts of the two deckrotary switch make contact with and electrically connect together thevarious fixed contacts in each of the S0 and S1 switching decks,respectively, of the two element rotary switch. From a consideration ofFIG. 4, it will be seen that at the zero power level setting, the S0deck serves to connect in circuit relationship the various capacitorelements (a) through (f) so that these capacitor elements comprise adesired amount of capacitance for the C0 shunting capacitor for eachoperating condition of the circuit. Similarly. the S] switching deckconnects in appropriate numbers of the capacitor elements (a) through(I) so as to comprise a predetermined amount ofCl commutatingcapacitance at a particular desired power output level. FIG. 4B of thedrawings constitutes a table listing which ones of the capacitorelements (a) through (f) are connected in circuit relationship throughtheir associated S0 and S] switching decks in order to comprise therequired amount of either C0 shunt capacitance or Cl commutatingcapacitance. From FIG. 4B, it will be seen that for power step l)capacitor elements ((1') and (e) will be connected in circuitrelationship through the S0 switching deck to form a desired amount ofshunt capacitance C0 and capacitor element (a) will be connected throughthe S1 switching deck to form the C1 commutating capacitance.

FIG. 4C of the drawings is an operating characteristic curveillustrating the operating characteristics of the power controllingcircuit depicted in FIG. 2 of the drawings which include the C0 and Clcapacitor switching circuit arrangement of FIGS. 4 and 4A. In FIG. 4Cpower output level is plotted against the turn off time t of the mainpower thyristor 01 used in the chopper inverter circuit 13. As shown inFIG. 4C there is a minimum vaiue of turn off time indicated as MIN tbelow which it is not safe to operate the chopper inverter due to thefact that insufficient off time will provide to assure saferecombination of the current carriers in the main power controllingthyristor Q1 so that it safely will regain its current blockingcapabilities intermediate each conduction interval. It is seen that thisMIN g time occurs at the nadir of the essentially U shape characteristiccurve wherein for increasing values of commutating capacitance Cl, poweroutput increases going from left to right from the nadir point; and thatfor increasing values of C0 shunt capacitance going from right to leftfrom the nadir point and decreasing values of power towards zero, the ttime of the circuit increases with increasing C0 values. Hence, it willbe seen that a wide range of power control level can be provided with aminimum number of capacitor elements wherein the capacitor elementsthrough an appropriate switching scheme can comprise either the C0 shuntcapacitance or the Cl commutating capacitance depending upon the settingof the switching ar rangement. Accordingly, wide range control of poweroutput level is achieved with a relatively simple and low cost capacitorbank and switching arrangement requiring a minimum number of capacitorelements.

FIG. 5 of the drawings is a series of current versus time characteristicwave shapes illustrating the thyristor SCR and diode current flowingthrough the chopper inverter SCR and diode pair for each of thedifferent power levels listed in the chart shown in FIG. 4B of thedrawings. In these characteristic wave shapes, time is plotted as theabscissa with each division block representing l0 microseconds andcurrent is plotted as the ordinate with each division mark representingabout 28 amperes per division. To the right of each of the currentversus time wave shapes the corresponding Cl commutating capacitance andC0 shunt capacitance values are listed together with the resulting I and0p erating periods that result from the particular Cl commutatingcapacitance and C capacitance values switched into operating circuitrelationship through appropriate setting of the SI and C0 switch decks.From a consideration of these values, as well as by observation of thecharacteristic wave shapes, it will be appreciated that the power thatcan be delivered by the circuit to the pan load by the induction heatingcoil increases for increasing values of commutating capacitance C1.

FIG. 6 is a detailed schematic circuit diagram of a lower cost versionof a new and improved induction cooking unit constructed in accordancewith the inven tion and which does not include the zero point soft turnon and turn off characteristic of the FIG. 2 circuit. Accordingly, thecircuit in FIG. 6 will exhibit higher levels of start and shut downvoltage and current transients; however, in certain applications, thischaracteristic is not objectional. For such applications, the lower costcircuit of FIG. 6 is preferred.

Many of the components of FIG. 6 correspond to the same numberedelements of the FIG, 2 circuit, and hence have been identified by thesame reference character. Accordingly, it is not believed necessary todescribe in detail the common similar parts of circuit construction inthe belief that the following description of operation will suffice topoint out the distinctions of FIG. 6 over the FIG. 2 circuit. To beginwith, the operators on-off switch S2 normally will be in the opencondition shown so that a positive polarity turn on potential issupplied through resistors R and R14 to the base of the Q5 switchingtransistor. Q5 and Q6 have their bases interconnected so that upon 05being turned on, both Q5 and Q6 latch on in the same manner as wasdescribed with reference to FIG. 2. Upon Q5 being turned on, no voltagecan be built up across the C5 timing capacitor of the gating pulsegenerator 21 due the clamp imposed across the timing capacitor throughthe diode D3 and O5 in the conducting condition. Hence, the cir cuit ismaintained in a clamped off condition. Upon the operators on off switchS2 being closed, the base of OS is placed at the volt potential level ofthe low voltage direct current power supply terminal and Q5 will bemaintained off. This will result in turning off Q6 so that both Q5 andQ6 are latched off, and will remove the clamp from across the C5 timingcapacitor in the gating pulse generator.

Upon the latching transistor Q6 being turned off, its collector willassume rapidly the negative potential of terminal 25 so that a negativegoing turn on pulse will be coupled through a capacitor C6 to the baseof the enabling transistor Q4 causing O4 to turn on and initiating theproduction of an initial gating on pulse from the gating pulse generator21. This initial gating pulse will be coupled through the pulsetransformer T1 having its primary connected in the cathode circuit ofthe PUT Q3 and its secondary connected to the gating electrode of thepilot SCR Q2. This results in gating on the main power control SCR O1and initiating oscilla tion of the chopper inverter as describedpreviously with reference to FIG. 2.

As the chopper inverter circuit 13 oscillates through one cycle ofoperation, the voltage appearing across the L3 induction heating coilwill be sensed and fed back by the feedback voltage dividing resistorsR2 and R3 and supplied through differentiating capacitor C4 to the baseof O4 in the manner described above with reference to FIG. 2.Consequently, the generation of gating pulses by the dv/dt t gatingpulse generator 21, will be sustained thereby causing operation of thechopper inverter circuit 13 to continue in the manner previouslydescribed. As loading and unloading of the L3 induction heating coiloccurs due to the placement and removal of a pan load 11, changes incommutating frequency due to tuning and detuning of the L3 inductionheating coil will again change the frequency or repetition rate of thegating pulses being generated by the gating pulse generator circuit 21to thereby cause the circuit to exhibit the above-described desiredfrequency pushing" characteristic that is necessary to allow it to beused safely with all types of pans including highly conductive pans ofcopper. aluminum, and the like. Changes in output power levels of thecircuit can be achieved with C0 and Cl switching arrangements similar tothat described in reference to FIG. 4 of the drawings although othertypes of power control are possible. It should be noted that whenswitching from one power level to the next, there is a mechanical orother type of interconnection between the S0 and S1 switch contacts andthe S2 operators on off switch, so that oscillations of the inverter arestopped during each switching operation. In this way, arcing anddeterioration of the S0181 switch contacts is prevented by not allowingthe switching to occur when high frequency, high voltage currents areflowing. For a more complete description of the construction of asuitable switch whereby an interlocking switching characteristic isachieved reference is made to the above-identified US. Pat. No.3,7l0.062.

It should be noted from the above description, that the point in thesupply voltage cycle where the gating generator 21 is enabled toinitiate operation of the main ()1 SCR and the chopper inverter circuit13, is in no way controlled relative to the zero points in the supplyvoltage and occur at any point in the cycle of the input supply voltage.Start-up transient levels will depend on the point in the cycle whereoscillations of the chopper inverter 13 are initiated. However, incertain applications, this characteristic may cause no problem, so thatthe added cost of the zero point pulse generator and zero pointcoincidence circuit can be avoided. In other aspects, the circuit ofFIG. 6 is entirely similar to that of FIG. 2 and possesses many of itsdesirable operating characteristics and features including thecapability for frequency pushing" with changes in loading.

With the circuit arrangement of FIG. 6, as well as that of FIG. 2, itmay also be desirable to include a metal oxide transient voltagesuppressor device ZNR connected across the power supply terminals 14 and15A for suppressing or limiting transient voltages appearing across thechopper inverter circuit to some predetermined maximum safe valuedepending upon the characteristics of the main power controlling SCR Q1.The metal oxide transient voltage suppressor device ZNR may comprise oneof the relatively new semiconductor voltage suppressor devices madeavailable in Japan by the Matsushita Electric Industrial Co. and in thiscountry by the Semiconductor Products Department of the General Electriccompany.

FIG. 6A of the drawings is a detailed schematic circuit diagram of amodification to the circuits shown in FIG. 6 and FIG. 2 whereby thecircuits can be adapted to use a pan temperature sensor for exactlycontrolling the temperature of the pan being inductively heated. A

preferred pan temperature sensor would comprise an infra red sensingdevice that looks directly at the pan being heated so as to maintain itstemperature at a pre cise set value and may be constructed in the mannerdescribed in US. Pat. No, 3,7l(),062. In a preferred form of the pantemperature sensing arrangement a conventional, commercial integratedcircuit operational amplifier is connected to operate as a bistablethreshold switch and receives its excitation power from across a pair ofvoltage dividing resistors R51 and R52 connected across the low voltagedirect current excita tion power supply 14, 25. The output of the op-ampthreshold switch supplied through load resistors R53 and R54 to the baseof a gating transistor Q31. Gating transistor Q31 is in effect connectedin series electrical circuit relationship with the operators on-offcontrol switch S2 so that when it is gated on and conducting itfunctions in precisely the same manner as S2 when it is closed to turnon the chopper inverter. This assumes that the S2 switch previously wasclosed by an operator of the equipment so that control over operation ofthe chopper inverter is surrendered to the temperature sensing amplifierarrangement. Conversely, upon the gating transistor Q31 being turned offby a positive output signal from threshold switch Sl, the end effectwill be the same as if the operator on off switch S2 were opened therebycausing the chopper inverter to turnoff. The output of threshold switch51 is designed to go positive only when the sensed pan temperatureexceeds a preset level. Hence, the pan temperature sensor controls ormodulates the induction cooking unit on and off in accordance with thesensed pan temperature. By this simple modification. the circuit of FIG.6, readily can be adapted to include the desirable pan temperaturesensing feature described previously in detail in U.S. Pat. No.3,710,062.

If incorporated into a complete cooking range with a number of otherinduction heating coils, each of the individual induction heating coilsL3 and its associated chopper inverter power supply and gatingcircuitry, will function in precisely the same manner as was describedwith relationship to FIG, 2 and/or FIG. 6 of the drawings. If desired,the particular induction heating coils L3 may be designed to provideeither a greater or smaller amount of power by appropriate design of thecoil to produce a larger or smaller induction magnetic field. As wasdescribed more fully in the above referenced US. Pat. No. 3,710,062 thisis accom plished by increasing or decreasing the number of turns inconstructing the induction heating coils, the size and diameter of thecoils, and of the wire used in forming the coils, and of course, thesize, voltage and current ratings of the chopper inverter power supplycomponents employed to excite the coils. Since it is believed thatindividual design of the various branch circuits to provide desiredlevels of output power in respective ones of the branches would beobvious to one skilled in the art, in view of the foregoing teachings, afurther description thereof is thought to be unnecessary.

From the foregoing description, it will be appreciated that the presentinvention provides new and improved induction cooking units which arecapable of safe operation with pans or other metal base cookware of alltypes including highly conductive pans made of aluminum, cooper, orother materials which advertently may be used by a housewife or otheroperator of the unit. The improved induction cooking unit is capable ofp eration over a wide number of output power levels including quite lowpower levels while providing ade quate commutating off time intervalsfor the SCR power thyristor used in the inverter circuit to excite theinduction heating coil thereby assuring that the commutating time offintervals are well within the rate of recovery time of the thyristor.Further, the circuit by appropriate design is capable of operation withthe production of a minimum of start up and shut down voltage andcurrent transients to assure safe operation. Having described severalembodiments of a new and improved induction heating unit for metal basecook ware according to the invention, it is believed obvious that othermodifications and variations of the invention are possible in the lightof the above teachings. It is therefore, to be understood that changesmay be made in the particular embodiments of the invention describedwhich are within the full intended scope of the invention as defined bythe appended claims.

What is claimed is:

1. An induction heating unit including in combination inverter circuitmeans comprising gate controlled thyristor means and commutation circuitmeans coupled together in circuit relationship and excited from a set ofpower supply terminals, an induction heating coil coupled to and excitedby said inverter circuit means in a manner such that the inductionheating coil determines at least in part the operating frequency atwhich the inverter circuit means operates, and gating circuit meanscoupled to and controlling turn-on of said gate controlled thyristormeans, said gating circuit means comprising feed back sensing circuitmeans coupled to said induction heating coil for deriving a feedbacktrigger signal synchronized with the frequency of operation of theinverter circuit circuit means, gating signal generator means forgenerating high frequency signal pulses having a repetition ratedetermined by the operation frequency for the inverter circuit means andof sufficient energy to insure turn-on of said gate controlled thyristormeans, enabling means coupled to and enabling initiation of operation ofsaid gating signal generator means, and alternating current signalcoupling circuit means intercoupling said last mentioned enabling meanswith said feedback sensing circuit means for synchronizing the operationof the gating signal generator means with changes in frequency of theinverter circuit means due to loading and unloading of the inductionheating coil.

2. An induction heating unit according to claim I wherein thealternating current signal coupling circuit means comprisesdifferentiating circuit means for differentiating the sensed value ofthe voltage appearing across the induction heating coil and supplyingthe same back to synchronize operation of the gating signal generatormeans with the changes in frequency of operation of the inverter circuitmeans due to loading and unloading of the induction heating coil.

3. An induction heating unit according to claim 1 wherein the invertercircuit means comprises a high frequency chopperinverter circuit meansincluding inductor and capacitor commutating reactive components havingan inductance Li and capacitance Cl. respectively, connected in seriescircuit relationship across the gate controlled thyristor means inparallel circuit relationship therewith and with the chopperinvertercircuit means thus comprised being connected across the set of powersupply terminals for connection to the source of excitation potentialthrough a filter inductor having an inductance L2, said commutatinginductor and capacitor being series resonant at a predetermined naturalcommutating frequency that provides a combined thyristor conduction andcommutating pe riod t during each cycle of operation and said gatingcircuit means controlling the turn-on of the gate controlled thyristormeans so as to render the thyristor conductive at a controlled frequencyof operation.

4. An induction heating unit according to claim 3 further including asmoothing inductor having an inductance L3 and a smoothing capacitorhaving capacitance C3 connected in series circuit relationship across atleast one of the capacitor and inductor commutating reactive components,said smoothing inductor and capacitor having values such that thecombined reactive impedance of the capacitor commutating reactivecomponent including the smoothing inductor and the smoothing capacitoris capacitive in nature and series resonates with the inductorcommutating component to establish the combined thyristorconductioncommutating period t and wherein the smoothing inductor andcapacitor shape the output current flowing through the smoothinginductor to a substantially sinusoidal wave shape having little or noradio frequency interference emission effects and improved powercoupling, and the smoothing inductor comprises the induction heatingcoil.

5. An induction heating unit according to claim 4 further includingshunt capacitor means and shunt capacitor switching means for switchingsaid shunt capacitor means in parallel circuit relationship across saidL3 induction heating coil at low power levels of operation for theinduction heating unit whereby the current flowing through the L3induction heating coil is reduced and the effective commutatingcapacitance C1 of the commutating capacitor is increased to therebyincrease the t, conduction and commutating time of the gate controlledthyristor means.

6. An induction heating unit according to claim 5 wherein thecommutating capacitance C] is comprised by a plurality of differentcapacitor elements and further includes C1 switching means for switchingin different combinations of capacitor elements in accordance with adesired power level at which the induction heating unit is to operate,and wherein the shunt capacitor means is formed by utilizing appropriateones of said capacitor elements switched into shunt circuitrelationships with L3 induction heating coil at low power levels ofoperation by the shunt capacitor switching means.

7. An induction heating unit according to claim 6 wherein the source ofexcitation potential for the induction heating unit comprises full waverectifier means designed for connection to a source of conventionalcommercial or residential alternating current and having the outputthereof connected across a filter capacitor of a relatively smallcapacitance value C2, said parallel connected series commutatedchopperinverter circuit means being connected through the L2 filterinductor across the filter capacitor and a metal oxide thyristortransient voltage supressor device connected in parallel circuitrelationsip across the chopper-inverter circuit means.

8. An induction heating unit according to claim 7, wherein the inductionheating coil comprises a planar, spirally round induction heating coil,a flat insulating support member for supporting inductively heatedcooking vessels disposed over the induction heating coil, electrostaticshield means formed on the under surface of the flat, insulating supportmember for electro-statically shielding the inductively heated cookingvessels from the induction heating coil, the electrostatic shieldingmeans being electrically grounded, and temperature responsive meanscoupled to sense the op erating temperature of at least the L3 inductionheating coil, and means coupling the output from the temperatureresponsive means to control the operation of said gating circuit meansto cause shut down of the induction heating unit upon anover-temperature condition being sensed.

9. An induction heating unit according to claim 8 wherein said gatingsignal generator means comprises a programable unijunction transistorhaving an anode, an anode-gate and a cathode, a timing capacitorconnected in parallel circuit relationship across the anodecathode ofthe programable unijunction transistor, output circuit means coupled tothe cathode of the programable unijunction transistor for supplying agatingon signal to said gate controlled thyristor means, adjustablecharging resistor means connected in common to the anode and to thetiming capacitor, biasing resistor means connected across the seriesconnected charging resistor and programable unijunction transistor andhaving an intermediate point connected to the anode-gate of theprogramable unijunction transistor, and terminal means connected acrossand biasing resistor means for supplying the gating signal generatormeans with a low voltage direct current excitation potential.

10. An induction heating unit according to claim 9 wherein the enablingmeans comprises transistor means having its emittor-collector connectedin series-circuit relationship with the parallel connected biasingresistor means and series connected charging resistor and timingcapacitor and programable unijunction transistor, and having its baseconnected to the output from feedback sensing circuit means.

11. An induction heating unit according to claim 10 further includingoperator controlled on-off switch means, and lock out circuit meanscontrolled by said operator controlled on-otf switch means and in turncoupled to and locking out operation of said gating signal generatormeans in response to the on-off condition of said operator controlledon-off switch means.

12. An induction heating unit according to claim 11 wherein the lockoutcircuit means comprises a set of opposite conductivity type transistorshaving the base of one transistor connected back to the emittorcollectorof the opposite conductivity type transistor so that both transistorslatch on or off in response to one of the transistors being turned on oroff, one of said latching opposite conductivity type transistors beingcoupled across the charging capacitor in the gating signal generatormeans, and one of the transistors having its base electrode connected tohave the highest switching potential applied thereto determined by thesetting of the operator controlled on-off switch means whereby positiveon-off switching of the clamp imposed across the charging capacitor inthe gating pulse signal generator means is achieved in response to onoffoperation of the operator controlled on-off switch means.

13. An induction heating unit according to claim 1 wherein said gatingsignal generator means comprises a programable unijunction transistorhaving an anode, an anode-gate and a cathode, a timing capacitorconnected in parallel circuit relationship across the anodecathode ofthe programable unijunction transistor, output circuit means coupled tothe cathode of the programable unijunction transistor for supplying agatingon signal to said gate controlled thyristor means, adjustablecharging resistor means connected in common to the anode and to thetiming capacitor, biasing resistor means connected across the seriesconnected charging resistor and programable unijunction transistor andhaving an intermediate point connected to the anode-gate of theprogramable unijunction transistor, and terminal means connected acrosssaid biasing resistor means for supplying the gating signal generatormeans with a low voltage direct current excitation potential.

14. An induction heating unit according to claim 13 wherein the enablingmeans comprises transistor means having its emittor-collector connectedin series-circuit relationship with the parallel connected biasingresistor means and series connected charging resistor and timingcapacitor and programable unijunction transistor, and having its baseconnected to the output from feedback sensing circuit means.

15. An induction heating unit according to claim 13 further includingoperator controlled on-off switch means, and lock out circuit meanscontrolled by said operator controlled on-off switch means and in turncoupled to and locking out operation of said gating signal generatormeans in response to the on-off condition of said operator controlledon-off switch means.

16. An induction heating unit according to claim 15 wherein the lockoutcircuit means comprises a set of opposite conductivity type transistorshaving the base of one transistor connected back to the emittorcollector of the opposite conductivity type transistor so that bothtransistors latch on or off in response to one of the transistors beingturned on or off, one of said latching opposite conductivity typetransistors being coupled across the charging capacitor in the gatingsig nal generator means, and one of the transistors having its baseelectrode connected to have the highest switching potential appliedthereto determined by the setting of the operator controlled on-offswitch means whereby positive on-off switching of the clamp imposedacross the charging capacitor in the gating pulse signal generator meansis achieved in response to onoff operation of the operator controlledon-off switch means.

17. An induction heating unit according to claim 1 further includingoperator controlled on-off switch means, and lock out circuit meanscontrolled by said operator controlled on-off switch means and in turncoupled to and locking out operation of said gating signal generatormeans in response to the on-off condition of said operator controlledon-off switch means.

18. An induction heating unit according to claim 1 wherein the source ofexcitation potential to which the power supply terminals are to beconnected provides a periodic undulating excitation potential that dropssubstantially to zero periodically, and the induction heating unitfurther includes zero point pulse generator means coupled to the powersupply terminals for producing a pulsed output signal at the occurrenceof each substantial zero point in the periodic undulating excitationpotential, and zero point coincidence circuit means coupled to andcontrolling operation of said enabling means and in turn coupled to andcontrolled by said zero point pulse generator means for conditioningsaid gating signal generator means for operation only at or near theperiodic zero point in the periodic undulating excitation potential.

19. An induction heating unit according to claim 18 wherein said zeropoint coincidence circuit means comprises a bilateral on-off latchingswitch circuit means having two stable states of operation and capableof being switched from one to the other of its stable states ofoperation only at or near a zero point in the periodic undulatingexcitation potential and inhibit circuit means intercoupling saidbilateral on-off latching switch circuit means to the gating signalgenerator means for inhibiting further operation of said gating signalgenerator means upon the bilateral on-off latching switch means beingswitched to an off condition only at or near a zero point of theperiodic undulating excitation potential.

20. An induction heating unit according to claim 18 further includingoperator controlled on-off switch means, and lock out circuit meanscontrolled by said operator controlled on-off switch means and in turncoupled to and locking out operating of said gating signal generatormeans in response to the on-off condition of said operator controlledon-off switch means.

2]. An induction heating unit according to claim 20 wherein the lockoutcircuit means comprises a set of opposite conductivity type transistorshaving the base of one transistor connected back to the emittercollectorof the opposite conductivity type transistor so that both transistorslatch on or off in response to one of the transistors being turned on oroff, one of said latching transistors being coupled to said bilateralonofflatching switch circuit means for enabling operation of thebilateral on-off latching switch circuit means at the next zero point ofthe periodic undulating excitation potential and having its baseelectrode connected to have a switching on-off potential applied theretoby the operator controlled on-off switch means whereby positive on-offswitching of the inhibit imposed on said gating pulse signal generatormeans by the inhibit circuit means is achieved in response to on-offoperation of the operator controlled on-off switch means only at or neara zero point of the supply undulating excitation potential.

22. An induction heating unit according to claim 20 wherein said gatingsignal generator means comprises a programable unijunction transistorhaving an anode, an anode-gate and a cathode, a timing capacitorconnected in parallel circuit relationship across the anodecathode ofthe programable unijunction transistor, output circuit means coupled tothe cathode of the programable unijunction transistor for supplying agatingon signal to said gate controlled thyristor means, adjustablecharging resistor means connected in common to the anode and to thetiming capacitor, biasing resistor means connected across the seriesconnected charging resistor and programable unijunction transistor andhaving an intermediate point connected to the anode-gate of theprogramable unijunction transistor and said enabling means comprisestransistor means having its emitter-collector connected in seriescircuit relationship with the parallel connected biasing resistor

1. An induction heating unit including in combination inverter circuitmeans comprising gate controlled thyristor means and commutation circuitmeans coupled together in circuit relationship and excited from a set ofpower supply terminals, an induction heating coil coupled to and excitedby said inverter circuit means in a manner such that the inductionheating coil determines at least in part the operating frequency atwhich the inverter circuit means operates, and gating circuit meanscoupled to and controlling turn-on of said gate controlled thyristormeans, said gating circuit means comprising feed back sensing circuitmeans coupled to said induction heating coil for deriving a feedbacktrigger signal synchronized with the frequency of operation of theinverter circuit circuit means, gating signal generator means forgenerating high frequency signal pulses having a repetition ratedetermined by the operation frequency for the inverter circuit means andof sufficient energy to insure turn-on of said gate controlled thyristormeans, enabling means coupled to and enablIng initiation of operation ofsaid gating signal generator means, and alternating current signalcoupling circuit means intercoupling said last mentioned enabling meanswith said feedback sensing circuit means for synchronizing the operationof the gating signal generator means with changes in frequency of theinverter circuit means due to loading and unloading of the inductionheating coil.
 2. An induction heating unit according to claim 1 whereinthe alternating current signal coupling circuit means comprisesdifferentiating circuit means for differentiating the sensed value ofthe voltage appearing across the induction heating coil and supplyingthe same back to synchronize operation of the gating signal generatormeans with the changes in frequency of operation of the inverter circuitmeans due to loading and unloading of the induction heating coil.
 3. Aninduction heating unit according to claim 1 wherein the inverter circuitmeans comprises a high frequency chopper-inverter circuit meansincluding inductor and capacitor commutating reactive components havingan inductance L1 and capacitance C1, respectively, connected in seriescircuit relationship across the gate controlled thyristor means inparallel circuit relationship therewith and with the chopper-invertercircuit means thus comprised being connected across the set of powersupply terminals for connection to the source of excitation potentialthrough a filter inductor having an inductance L2, said commutatinginductor and capacitor being series resonant at a predetermined naturalcommutating frequency that provides a combined thyristor conduction andcommutating period t1 during each cycle of operation and said gatingcircuit means controlling the turn-on of the gate controlled thyristormeans so as to render the thyristor conductive at a controlled frequencyof operation.
 4. An induction heating unit according to claim 3 furtherincluding a smoothing inductor having an inductance L3 and a smoothingcapacitor having capacitance C3 connected in series circuit relationshipacross at least one of the capacitor and inductor commutating reactivecomponents, said smoothing inductor and capacitor having values suchthat the combined reactive impedance of the capacitor commutatingreactive component including the smoothing inductor and the smoothingcapacitor is capacitive in nature and series resonates with the inductorcommutating component to establish the combined thyristorconduction-commutating period t1, and wherein the smoothing inductor andcapacitor shape the output current flowing through the smoothinginductor to a substantially sinusoidal wave shape having little or noradio frequency interference emission effects and improved powercoupling, and the smoothing inductor comprises the induction heatingcoil.
 5. An induction heating unit according to claim 4 furtherincluding shunt capacitor means and shunt capacitor switching means forswitching said shunt capacitor means in parallel circuit relationshipacross said L3 induction heating coil at low power levels of operationfor the induction heating unit whereby the current flowing through theL3 induction heating coil is reduced and the effective commutatingcapacitance C1 of the commutating capacitor is increased to therebyincrease the t1 conduction and commutating time of the gate controlledthyristor means.
 6. An induction heating unit according to claim 5wherein the commutating capacitance C1 is comprised by a plurality ofdifferent capacitor elements and further includes C1 switching means forswitching in different combinations of capacitor elements in accordancewith a desired power level at which the induction heating unit is tooperate, and wherein the shunt capacitor means is formed by utilizingappropriate ones of said capacitor elements switched into shunt circuitrelationships with L3 induction heating coil at low power levels ofoperation by the shunt capacitor switching means.
 7. An inductionheating unit according to claim 6 wherein the source of excitationpotential for the induction heating unit comprises full wave rectifiermeans designed for connection to a source of conventional commercial orresidential alternating current and having the output thereof connectedacross a filter capacitor of a relatively small capacitance value C2,said parallel connected series commutated chopper-inverter circuit meansbeing connected through the L2 filter inductor across the filtercapacitor and a metal oxide thyristor transient voltage supressor deviceconnected in parallel circuit relationsip across the chopper-invertercircuit means.
 8. An induction heating unit according to claim 7,wherein the induction heating coil comprises a planar, spirally roundinduction heating coil, a flat insulating support member for supportinginductively heated cooking vessels disposed over the induction heatingcoil, electrostatic shield means formed on the under surface of theflat, insulating support member for electro-statically shielding theinductively heated cooking vessels from the induction heating coil, theelectrostatic shielding means being electrically grounded, andtemperature responsive means coupled to sense the operating temperatureof at least the L3 induction heating coil, and means coupling the outputfrom the temperature responsive means to control the operation of saidgating circuit means to cause shut down of the induction heating unitupon an over-temperature condition being sensed.
 9. An induction heatingunit according to claim 8 wherein said gating signal generator meanscomprises a programable unijunction transistor having an anode, ananode-gate and a cathode, a timing capacitor connected in parallelcircuit relationship across the anode-cathode of the programableunijunction transistor, output circuit means coupled to the cathode ofthe programable unijunction transistor for supplying a gating-on signalto said gate controlled thyristor means, adjustable charging resistormeans connected in common to the anode and to the timing capacitor,biasing resistor means connected across the series connected chargingresistor and programable unijunction transistor and having anintermediate point connected to the anode-gate of the programableunijunction transistor, and terminal means connected across and biasingresistor means for supplying the gating signal generator means with alow voltage direct current excitation potential.
 10. An inductionheating unit according to claim 9 wherein the enabling means comprisestransistor means having its emittor-collector connected inseries-circuit relationship with the parallel connected biasing resistormeans and series connected charging resistor and timing capacitor andprogramable unijunction transistor, and having its base connected to theoutput from feedback sensing circuit means.
 11. An induction heatingunit according to claim 10 further including operator controlled on-offswitch means, and lock out circuit means controlled by said operatorcontrolled on-off switch means and in turn coupled to and locking outoperation of said gating signal generator means in response to theon-off condition of said operator controlled on-off switch means.
 12. Aninduction heating unit according to claim 11 wherein the lockout circuitmeans comprises a set of opposite conductivity type transistors havingthe base of one transistor connected back to the emittor-collector ofthe opposite conductivity type transistor so that both transistors latchon or off in response to one of the transistors being turned on or off,one of said latching opposite conductivity type transistors beingcoupled across the charging capacitor in the gating signal generatormeans, and one of the transistors having its base electrode connected tohave the highest switching potential applied thereto determined by thesetting of the operator controlled on-off switch means whereby positiveon-off switching of the clamp imposed acrosS the charging capacitor inthe gating pulse signal generator means is achieved in response toon-off operation of the operator controlled on-off switch means.
 13. Aninduction heating unit according to claim 1 wherein said gating signalgenerator means comprises a programable unijunction transistor having ananode, an anode-gate and a cathode, a timing capacitor connected inparallel circuit relationship across the anode-cathode of theprogramable unijunction transistor, output circuit means coupled to thecathode of the programable unijunction transistor for supplying agating-on signal to said gate controlled thyristor means, adjustablecharging resistor means connected in common to the anode and to thetiming capacitor, biasing resistor means connected across the seriesconnected charging resistor and programable unijunction transistor andhaving an intermediate point connected to the anode-gate of theprogramable unijunction transistor, and terminal means connected acrosssaid biasing resistor means for supplying the gating signal generatormeans with a low voltage direct current excitation potential.
 14. Aninduction heating unit according to claim 13 wherein the enabling meanscomprises transistor means having its emittor-collector connected inseries-circuit relationship with the parallel connected biasing resistormeans and series connected charging resistor and timing capacitor andprogramable unijunction transistor, and having its base connected to theoutput from feedback sensing circuit means.
 15. An induction heatingunit according to claim 13 further including operator controlled on-offswitch means, and lock out circuit means controlled by said operatorcontrolled on-off switch means and in turn coupled to and locking outoperation of said gating signal generator means in response to theon-off condition of said operator controlled on-off switch means.
 16. Aninduction heating unit according to claim 15 wherein the lockout circuitmeans comprises a set of opposite conductivity type transistors havingthe base of one transistor connected back to the emittor-collector ofthe opposite conductivity type transistor so that both transistors latchon or off in response to one of the transistors being turned on or off,one of said latching opposite conductivity type transistors beingcoupled across the charging capacitor in the gating signal generatormeans, and one of the transistors having its base electrode connected tohave the highest switching potential applied thereto determined by thesetting of the operator controlled on-off switch means whereby positiveon-off switching of the clamp imposed across the charging capacitor inthe gating pulse signal generator means is achieved in response toon-off operation of the operator controlled on-off switch means.
 17. Aninduction heating unit according to claim 1 further including operatorcontrolled on-off switch means, and lock out circuit means controlled bysaid operator controlled on-off switch means and in turn coupled to andlocking out operation of said gating signal generator means in responseto the on-off condition of said operator controlled on-off switch means.18. An induction heating unit according to claim 1 wherein the source ofexcitation potential to which the power supply terminals are to beconnected provides a periodic undulating excitation potential that dropssubstantially to zero periodically, and the induction heating unitfurther includes zero point pulse generator means coupled to the powersupply terminals for producing a pulsed output signal at the occurrenceof each substantial zero point in the periodic undulating excitationpotential, and zero point coincidence circuit means coupled to andcontrolling operation of said enabling means and in turn coupled to andcontrolled by said zero point pulse generator means for conditioningsaid gating signal generator means for operation only at or near theperiodic zero point in the periodic undulating excitation potential. 19.An induction heating unit according to claim 18 wherein said zero pointcoincidence circuit means comprises a bilateral on-off latching switchcircuit means having two stable states of operation and capable of beingswitched from one to the other of its stable states of operation only ator near a zero point in the periodic undulating excitation potential andinhibit circuit means intercoupling said bilateral on-off latchingswitch circuit means to the gating signal generator means for inhibitingfurther operation of said gating signal generator means upon thebilateral on-off latching switch means being switched to an offcondition only at or near a zero point of the periodic undulatingexcitation potential.
 20. An induction heating unit according to claim18 further including operator controlled on-off switch means, and lockout circuit means controlled by said operator controlled on-off switchmeans and in turn coupled to and locking out operating of said gatingsignal generator means in response to the on-off condition of saidoperator controlled on-off switch means.
 21. An induction heating unitaccording to claim 20 wherein the lockout circuit means comprises a setof opposite conductivity type transistors having the base of onetransistor connected back to the emitter-collector of the oppositeconductivity type transistor so that both transistors latch on or off inresponse to one of the transistors being turned on or off, one of saidlatching transistors being coupled to said bilateral on-off latchingswitch circuit means for enabling operation of the bilateral on-offlatching switch circuit means at the next zero point of the periodicundulating excitation potential and having its base electrode connectedto have a switching on-off potential applied thereto by the operatorcontrolled on-off switch means whereby positive on-off switching of theinhibit imposed on said gating pulse signal generator means by theinhibit circuit means is achieved in response to on-off operation of theoperator controlled on-off switch means only at or near a zero point ofthe supply undulating excitation potential.
 22. An induction heatingunit according to claim 20 wherein said gating signal generator meanscomprises a programable unijunction transistor having an anode, ananode-gate and a cathode, a timing capacitor connected in parallelcircuit relationship across the anode-cathode of the programableunijunction transistor, output circuit means coupled to the cathode ofthe programable unijunction transistor for supplying a gating-on signalto said gate controlled thyristor means, adjustable charging resistormeans connected in common to the anode and to the timing capacitor,biasing resistor means connected across the series connected chargingresistor and programable unijunction transistor and having anintermediate point connected to the anode-gate of the programableunijunction transistor and said enabling means comprises transistormeans having its emitter-collector connected in series circuitrelationship with the parallel connected biasing resistor means and theseries connected charging resistor and parallel connected timingcapacitor and programable unijunction transistor, and having its baseconnected to the output from the feedback sensing circuit means.
 23. Aninduction heating unit according to claim 22 wherein the alternatingcurrent signal coupling circuit means comprises differentiating circuitmeans for differentiating the sensed value of the voltage appearingacross the induction heating coil and supplying the same back tosynchronize operation of the gating signal generator means with thechanges in frequency of operation of the inverter circuit means due toloading and unloading of the induction heating coil.
 24. An inductionheating unit according to claim 23 wherein the inverter circuit meanscomprises a high frequency chopper-inverter circuit means includinginductor and capacitor commutating reactive components having aninductance L1 and cApacitance C1, respectively, connected in seriescircuit relationship across the gate controlled thyristor means inparallel circuit relationship therewith and with the chopper-invertercircuit means thus comprised being connected across the set of powersupply terminals for connection to the source of excitation potentialthrough a filter inductor having an inductance L2, said commutatinginductor and capacitor being tuned to series resonance at apredetermined natural commutating frequency that provides a combinedthyristor conduction and commutating period t1 during each cycle ofoperation and said gating circuit means controlling the turn-on of thegate controlled thyristor means so as to render the thyristor conductiveat a controlled frequency of operation that provides an operation periodT for the chopper-inverter circuit means including a quiescent chargingperiod t2 in each cycle of operation where T t1 + t2 such that the valueomega 2t2 equals substantially pi /2 radians at the operating frequencyor greater and where omega 2 equals 1/ Square Root L2C1 whereby thereapplied forward voltage across the thyristor means following eachconduction interval is maintained substantially independent of load. 25.An induction heating unit according to claim 24 further including asmoothing inductor having an inductance L3 and a smoothing capacitorhaving capacitance C3 connected in series circuit relationship across atleast one of the capacitor and inductor commutating reactive components,said smoothing inductor and capacitor having values such that thecombined reactive, impedance of the capacitor commutating reactivecomponent including the smoothing inductor and the smoothing capacitoris capacitive in nature and series resonates with the inductorcommutating components to establish the combined thyristorconduction-commutating period t1, and wherein the smoothing inductor andcapacitor shape the output current flowing through the smoothinginductor to a substantially a sinusoidal wave shape having little or noradio frequency interference emission effects and improved powercoupling, and the smoothing inductor comprises the induction heatingoil.
 26. An induction heating unit according to claim 25 furtherincluding shunt capacitor means and shunt capacitor switching means forswitching said shunt capacitor means in parallel circuit relationshipacross said L3 induction heating coil at low power levels of operationfor the induction heating unit whereby the current flowing through theL3 induction heating coil is reduced and the effective commutatingcapacitance C1 of the commutating capacitor is increased to therebyincrease the t1 conduction and commutating time of the gate controlledthyristor means.
 27. An induction heating unit according to claim 26wherein the commutating capacitance C1 is comprised by a plurality ofdifferent capacitor elements and further includes C1 switching means forswitching in different combinations of capacitor elements in accordancewith a desired power level at which the induction heating unit is tooperate, and wherein the shunt capacitor means is formed by utilizingappropriate ones of said capacitor elements switched into shunt circuitrelationships with L3 induction heating coil at low power levels ofoperation by the shunt capacitor switching means.
 28. An inductionheating unit according to claim 27 wherein the source of excitationpotential for the induction heating unit comprises full wave rectifiermeans designed for connection to a source of conventional commercial orresidential alternating current and having the output thereof connectedacross a filter capacitor of a relatively small capacitance value C2,said parallel connected series commutated chopper-inverter circuit meansbeing connected through the L2 filter inductor across the filtercapacitor and a metal oxide thyristor transient voltagE supressor deviceconnected in parallel circuit relationship across the chopper-invertercircuit means.
 29. An induction heating unit according to claim 28wherein the induction heating coil comprises a planar, spirally woundinduction heating coil, a flat insulating support member for supportinginductively heated cooking vessels disposed over the induction heatingcoil, electrostatic shield means formed on the under surface of theflat, insulating support member for electro-statically shielding theinductively heated cooking vessels from the induction heating coil, theelectrostatic shielding means being electrically grounded, andtemperature responsive means coupled to sense the operating temperatureof at least the L3 induction heating coil, and means coupling the outputfrom the temperature responsive means to control the operation of saidgating circuit means to cause shut down of the induction heating unitupon an over-temperature condition being sensed.